1. Field of the Invention
The present invention relates to a resonant-type switching power supply apparatus including a first switching element and a second switching element which are located on the primary side of a transformer and are alternately turned on and off.
2. Description of the Related Art
Currently, resonant-type switching power supply apparatuses are used to achieve high circuit efficiency (see, for example, Japanese Unexamined Patent Application Publication Nos. 2002-112544 and 2002-209381). FIG. 1 is a circuit diagram of a switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-112544.
In a switching power supply apparatus illustrated in FIG. 1, a primary winding T1 of a transformer T, an inductor Lr, and a capacitor Cr are connected in series and form a series circuit. One end of this series circuit is connected to a node between a first switching circuit S1 including a first switching element Q1 and a second switching circuit S2 including a second switching element Q2, and the other end thereof is connected to a power input portion.
A first driving winding T3 of the transformer T generates a voltage substantially proportional to a voltage generated by the primary winding T1. The generated voltage is input into a first control circuit 11. The first control circuit 11 includes: a delay circuit that is a series circuit including a resistor R3 and a capacitor C3 and connected between the first driving winding T3 and a gate of the first switching element Q1; a transistor Tr1 that is a switching portion for turning off the first switching element Q1; and a time constant circuit that includes a capacitor C4 and a photocoupler PC for receiving a feedback signal from a detection circuit 14 and is connected to a base of the transistor Tr1. The control circuit 11 turns on the first switching element Q1 with a delay after the first driving winding T3 has generated a voltage. Furthermore, the control circuit 11 turns off the first switching element Q1 quickly by turning on the transistor Tr1 after a time set by the time constant circuit, which has the impedance of the photocoupler PC and the capacitor C4, has elapsed since the primary winding T3 generated a voltage.
The transformer T includes a second driving winding T4. A voltage generated by the second driving winding T4 is applied to a second control circuit 12. The second control circuit 12 includes: a delay circuit that is a series circuit including a resistor R5 and a capacitor C5 and connected in series to the second driving winding T4; a transistor Tr2 that is a switching portion for turning off the second switching element Q2; and a time constant circuit that includes a resistor R6 and a capacitor C6 and is connected to a base of the transistor Tr2.
On the side of a secondary winding T2 of the transformer T, a rectifier diode Ds, a smoothing capacitor Co, and an output voltage detection circuit 14 are disposed.
Thus, the first switching element Q1 is turned on using an AC voltage generated by the driving winding T3, and the second switching element Q2 is turned on using an AC voltage generated by the driving winding T4. Furthermore, the first switching element Q1 and the second switching element Q2 are turned off by the first control circuit 11 and the second control circuit 12, respectively, each of which operates using an AC voltage.
FIG. 2 is a circuit diagram of a switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-209381. As illustrated in FIG. 2, a DC power supply 30 is connected in parallel to a series circuit including an FET 21 and an FET 22. A series circuit including a capacitor 23 and a primary winding 25 of a transformer 24 is connected in parallel to the FET 22. On the secondary side of the transformer 24, two windings (a winding 28 and a winding 29) and a rectifying and smoothing circuit including a diode 32, a diode 33, and a capacitor 34 are disposed. In order to maintain a smoothed DC output voltage constant, an output voltage detection circuit 37 performs feedback control. A tertiary winding 26 of the transformer 24 is connected to a gate of the FET 22 via a resistor 36. A diode 31 and a capacitor 35 perform half-wave rectification upon a voltage generated by a quaternary winding 27 of the transformer 24. A voltage acquired by the half-wave rectification is used as a power supply voltage for a control circuit 38. The quaternary winding 27 of the transformer 24 is connected to the control circuit 38 so as to detect the switching of the power supply voltage.
Thus, the FET 21 (a first switching element) is driven and controlled by a logic circuit that operates using a DC voltage, and the FET 22 (a second switching element) is turned on and off using an AC voltage generated by the tertiary winding 26 (driving winding).
However, in the switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-112544, the first switching element Q1 is driven, using a voltage generated by the driving winding T3 for generating a voltage substantially proportional to a voltage generated by the primary winding, by a circuit that operates using an AC voltage generated by the driving winding T3. Accordingly, a logic circuit that operates using a DC voltage cannot be used. It is necessary to use a circuit that operates using an AC voltage for each of an overcurrent protection operation, an overvoltage protection operation, a startup operation, and a stop operation. This leads to a complicated circuit configuration and makes the miniaturization of a switching power supply apparatus difficult.
Furthermore, it is difficult to integrate circuits, each of which operates using an AC voltage. This makes the miniaturization of a control circuit difficult.
In the switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2002-209381, the FET 22 is driven and turned on and off using a voltage generated by the tertiary winding 26. Accordingly, an ON period of the FET 22 cannot be set, and a switching frequency cannot be adjusted. Such a switching power supply apparatus cannot therefore efficiently operate in response to changes in input voltage or load. Furthermore, there are the following disadvantages in such a switching power supply apparatus. The configurations of circuits used for an overcurrent protection operation, an overvoltage protection operation, a startup operation, and a stop operation become complicated. When circuits operate, a power loss is increased due to temperature characteristics of circuit components and variations in characteristics of the circuit components. Such a switching power supply apparatus becomes larger in size.